Carry Propagation

In the last page, we have left the carry lever in the 'warned' state, that is, the
bottom figure wheel has crossed from 9 to 0, so a carry is due in the wheel right above
it.

While detecting the carry, the lever was in such a position that the curved arm could
be engaged by any of the four decade teeth, but the upper forked arm did not engage the
upper wheel. At that time, the upper wheel would be receiving its own addition so it
has to be left free to do so.

Of course, there are other carry levers, not shown, both above and below the one
shown, detecting and processing the carries of each digit to the next.

After the addition happens, the carry arm moves vertically so that both arms can engage
the two wheels, the curved arm is still at the height of the decade teeth, while the upper
forked arm is at the height of the units teeth of the figure wheel above, just as can be
seen in the static image above.

Now, we change the point of view looking almost from above towards the same carry
lever. Another device can be seen to the left, in blue. It is the carry drive
arm. There is one for each carry lever and they are the ones that move the carry
arms if they have detected a carry.

The sequence starts when the sector wheels (in magenta) are adding a number to the
figure wheels in yellow. At some point, the green carry lever is pushed by the
bottom figure wheel. The sector and figure wheels then stop, since the addition
phase is finished, and the sector wheel moves half way up so it disengages from the
receiving figure wheels, as mentioned in Restoring the Values.

The carry lever also moves to its new position and at some point, the carry drive
starts turning and the arm hits the carry lever, making it turn a little bit more.
Since at this point, the upper forked arm is engaged with the upper figure wheel, it
carries it along one unit.

To finish the cycle, which is not shown here, the carry drive makes a full turn until
it comes back to its resting position. The carry lever also moves up so it
disengages from all wheels and is pushed to its 'unwarned' original position.